The present invention relates to a hydraulic control system for a hydrostatically operated earth-working machine having plural hydraulic circuits, and more particularly, to a control system for controlling ground speed, forward and reverse movement, steering, and braking for the machine.
It is conventional to provide a hydrostatically operated earthworking machine having separate hydraulic systems, including one for powering the ground drive, another for providing steering, and still another for powering the implement attachment. Typically, the operator manually adjusts the ground speed of the machine depending upon the load resistance being encountered by the implement. This manual correlation between ground speed and implement load is intended to maintain a constant load on the machine and thereby provide optimum engine efficiency.
A problem with known hydraulically operated tractor-implement systems is that the operator must constantly monitor the load conditions of the implement and simultaneously adjust the ground speed in response thereto. In practice, it is nearly impossible for the operator to maintain optimum engine efficiency and still perform the other operations required of him in running the machine.
For example, in known hydrostatically operated trenchers, a trenching boom is mounted to the tractor and one hydraulic system powers the digging chain on the boom while other hydraulic systems control ground speed, forward and reverse movement, steering, and braking. If the powered digging chain suddenly encounters a rocky soil condition, the operator must slow the tractor until the higher pressure developed in the trencher hydraulic circuit, due to the increased load, is reduced either by the trencher passing through the area or by some operator action. At the same time the operator is attempting to maintain maximum tractor engine efficiency, he must steer the unit and perform other normal running operations.
A conventional manual control, generally known as a mono stick assembly, allows the operator to use only one manually operable handle to control ground speed, forward and reverse movement, steering, aand braking for the machine. As would be expected, the mono stick control assembly has improved the operator's ability to perform the operations required of him in running the machine. However, known constructions of mono stick assemblies are relatively complex and expensive and generally custom designed for the particular machine on which they are being used. Thus, there has been a need for an improved mono stick assembly which is simpler, less expensive, and adaptable for use in various machines while still combining the control for several machine functions into one assembly.
A further problem with conventional manual controls, including the mono stick type, is that they are not always effective in maintaining high engine efficiency during high load conditions because it is not always possible for the operator to give his undivided attention to monitoring the implement load and ground speed variables. Thus, there has been a need for an additional ajustable control associated with the mono stick assembly which can be selectively set independently of the mono stick assembly during high load, low machine speed conditions without requiring the operator to constantly manipulate the mono stick control.
The disadvantages of present hydrostatically operated earthworking machines have resulted in the hydraulic control system of the present invention which effectively reduces required operator participation and results in greater output because the tractor engine may be controlled to operate at substantially optimum efficiency, particularly during high load, low machine speed operations.